Nowadays natural rubber (one example of polyisoprenoids) for use in industrial rubber products are obtained by cultivating rubber-producing plants, such as para rubber tree (Hevea brasiliensis) belonging to the family Euphorbiaceae, or Indian rubber tree (Ficus elastica) belonging to the family Moraceae, to biosynthesize natural rubber by the lactiferous cells of the plants, and harvesting the natural rubber by hand from the plants.
At present, Hevea brasiliensis is practically the only one source of natural rubber for industrial rubber products. Hevea brasiliensis is a plant that can grow only in limited areas such as in Southeast Asia and South America. Conventionally, natural rubber production has been increased by applying ethephon or methyl jasmonate to Hevea brasiliensis trees to induce lactiferous duct formation. Moreover, Hevea brasiliensis requires about seven years from planting to mature enough for rubber extraction, and the period during which natural rubber can be extracted is limited to 20 to 30 years. Although more natural rubber is expected to be demanded mainly by developing countries in years to come, for the reason mentioned above it is difficult to greatly increase the production of natural rubber using Hevea brasiliensis. Depletion of natural rubber sources is therefore of concern, and there are needs for stable natural rubber sources other than mature Hevea brasiliensis and for improvement in productivity of natural rubber from Hevea brasiliensis. 
Natural rubber has a cis-1,4-polyisoprene structure formed mainly of isopentenyl diphosphate (IPP) units, and the nature of this structure suggests that cis-prenyltransferase (CPT) is involved in natural rubber biosynthesis. For example, several CPTs are found in Hevea brasiliensis, including Hevea rubber transferase 1 (HRT1) and Hevea rubber transferase 2 (HRT2) (see for example Non Patent Literatures 1 and 2). It is also known that rubber synthesis can be reduced in the dandelion species Taraxacum brevicorniculatum by suppressing CPT expression (see for example Non Patent Literature 3).
Previous studies of proteins associated with natural rubber biosynthesis have focused on rubber elongation factor (REF) and small rubber particle protein (SRPP) (see for example Non Patent Literatures 4 and 5). However, the associations between these proteins and CPT are not completely understood.
It has also been suggested that Nogo-B receptor (NgBR) is involved in dolichol biosynthesis by a human CPT (see for example Non Patent Literature 6).
Methods have been studied for increasing the production of natural rubber in Hevea brasiliensis, but since the rubber synthesis mechanism of Hevea brasiliensis is not completely understood, Hevea brasiliensis variants have been proposed which have been genetically modified to express and enhance genes of the known monomer (isopentenyl diphosphate) synthesis pathways (mevalonate (MVA) pathway and non-mevalonate (MEP) pathway) (see for example Patent Literature 1).